The present invention relates to a brilliance control circuit for automatically controlling the brilliance of fluorescent display tubes.
As shown in FIGS. 3 and 4, the conventional fluorescent display tube lighting circuit employs a Zener diode ZD1 to maintain the lighting voltage on a fixed level. FIG. 5 is a waveform chart of grid voltage regulated by a circuit shown in FIG. 3, and FIG. 6 is a waveform chart of grid voltage regulated by a circuit shown in FIG. 4.
However, when such a conventional lighting circuit is used in combination with a plurality of fluorescent display tubes, the respective inherent brilliances of the fluorescent display tubes are different from each other. Accordingly, when a plurality of fluorescent display tubes are used in combination to display a numerical value of a plurality of digits, the digits are different from each other in brilliance, which makes reading the numerical value very difficult.